Insulated wires, cables, cords, optical fiber core wires, used as inside or outside wiring for electric and electronic instruments, optical fiber cord and the like, are required to have various properties such as flame retardancy, heat resistance, and mechanical characteristics (for example, tensile properties and abrasion resistance).
As the materials for these wiring materials, use is made of resin compositions prepared by incorporating the metal hydrate such as magnesium hydroxide or aluminum hydroxide in large quantities.
In addition, the temperatures of the wiring materials used for electric or electronic instruments may rise to from 80 to 105° C., even to about 125° C., under continuous use, so that heat resistance is required in some applications. In such a case, high heat resistance is imparted to the wiring materials by applying a method of crosslinking a coating material by electron beam crosslinking or chemical crosslinking.
So far, as a method of crosslinking a polyolefin resin such as polyethylene, or rubber such as ethylene-propylene rubber or chloroprene rubber, an electron beam crosslinking method of irradiating with electron beams to cause bridging (also referred to as crosslinking), a chemical crosslinking method of applying heat, after molding, to decompose organic peroxide or the like to allow a crosslinking reaction, and a silane crosslinking method have been known.
Among these crosslinking methods, because in most cases silane crosslinking methods do not particularly require special facilities, they can therefore be used in a wide variety of fields.
The silane crosslinking method is a method of obtaining a crosslinked molded body, by a grafting reaction of a silane coupling agent having an unsaturated group onto a polymer in the presence of organic peroxides, to obtain a silane graft polymer, and then contacting the silane graft polymer with water in the presence of a silanol condensation catalyst.
To give a concrete example, as a method of producing a halogen-free heat-resistant silane crosslinked resin, there is a method of melt-blending a silane master batch prepared by grafting a hydrolyzable silane coupling agent having an unsaturated group onto a polyolefin resin, a heat-resistant master batch prepared by kneading a polyolefin resin and an inorganic filler, and a catalyst master batch containing a silanol condensation catalyst.
However, in this method, when the inorganic filler exceeds 100 parts by mass with respect to 100 parts by mass of the polyolefin resin, it becomes difficult to conduct uniform melt-kneading thereof in a single-screw extruder or a twin-screw extruder, after the silane master batch and the heat-resistant master batch are dry mixed. This causes problems such as deterioration of appearance, significant degeneration of physical properties, and difficulty of molding with high extrusion load.
Accordingly, in performing dry blending of the silane master batch with the heat-resistant master batch, and then uniformly melt-kneading them, a ratio of the inorganic filler is restricted, as mentioned above. Therefore, it has been difficult to achieve high flame retardancy and high heat resistance.
Generally, for the kneading in the case where the inorganic filler exceeds 100 parts by mass with respect to 100 parts by mass of polyolefin resin, an enclosed mixer such as a continuous kneader, a pressurized kneader, or a Banbury mixer is generally used.
In the meantime, when a silane grafting reaction is performed in a kneader or a Banbury mixer, the hydrolyzable silane coupling agent having an unsaturated group, which generally has high volatility, volatizes before grafting reaction. Therefore, it was very difficult to prepare a desired silane crosslinking master batch.
Therefore, in the case of preparing a heat-resistant silane master batch with a Banbury mixer or a kneader, consideration might be given to a method which includes adding organic peroxides and a silane coupling agent having a hydrolysable unsaturated group to the heat-resistant master batch prepared by melt-blending a polyolefin resin and an inorganic filler, and then subjecting the resultant to graft-reaction in a single-screw extruder.
However, according to such a method, defects in the appearance of molded body would sometimes occur due to uneven reaction. Further, the need to incorporate a very large amount of inorganic filler in the master batch would sometimes result in very high extrusion load. These make it very difficult to manufacture a molded body. As a result, it was difficult to obtain a desired material or molded body. In addition, the method involves two steps and therefore has a big problem in terms of cost.
Patent Literature 1 proposes a method in which an inorganic filler surface-treated with a silane coupling agent, a silane coupling agent, an organic peroxide, and a crosslinking catalyst are thoroughly melt-kneaded with a kneader into a resin component formed by mixing a polyolefin-based resin and a maleic anhydride-based resin, and then the blend is molded with a single-screw extruder.
In addition, Patent Literatures 2 to 4 propose a method of partially crosslinking a vinyl aromatic thermoplastic elastomer composition prepared by adding a non-aromatic softener for rubber as a softener, to a block copolymer or the like as a base resin, through a silane surface-treated inorganic filler using organic peroxide.
Further, Patent Literature 5 proposes a method in which organic peroxide, a silane coupling agent, and a metal hydrate are melt-kneaded with a base material in batch, and further melt-molded together with a silanol condensation catalyst, and then crosslinked in the presence of water, to easily obtain a cable having heat resistance.